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Sequence reveals genes behind bizarre sea-horse traits

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The gods of Greek mythology were busy people. Poseidon, as well as having dominion over the sea and sending earthquakes, had a sideline in creating animals. His most celebrated design was the horse. Poseidon was so keen on his horses that he held onto some to pull his chariot through the waves. These first sea horses — called the hippocampi or, loosely, horse-monsters — had the tails of fish and two front hooves. They could be seen on a windy day, racing across the foam and waves of the sea’s surface. That’s why ocean breakers are still called white horses.

The sea horse, in other words — or its name at least — has a complicated origin story. Whereas Poseidon’s mythical horses were considered the most beautiful creatures of the ancient world, the real sea horse has a tale of wonder of its own to tell. These fragile, elegant animals look like almost nothing else on Earth (except, naturally, a horse, and a distinctive part of the human brain). They are fish without scales and the usual fins. They are covered in bony plates. They swim upright. They form monogamous pairs. And most famously, the male sea horse experiences pregnancy — well, the closest that fish get to pregnancy — as he holds and nurtures the developing embryo in a special pouch.

In a paper this week, scientists explore the bizarre features of the sea horse from the inside out (Q. Lin et al. Nature 540, 395–399; 2016). They describe how they sequenced and analysed the genome of Hippocampus comes, the tiger tail sea horse (just to add to the morphological mix). The results offer some clues to the genetic basis of their unique traits.

A gene family with a role in embryo hatching shows high expression in the male brood pouch, the scientists say. And some potential regulatory elements are missing, which might help to explain the evolution of the sea horse’s strange body shape. The animals eat through a tubular snout (no teeth) and, sure enough, the genome showed a lack of genes for enamel proteins, needed to make teeth. The absence of a gene called tbx4, a known regulator of limb development, may have contributed to the loss of pelvic fins. And to the unusual features of the sea horse we can add a relatively high evolutionary rate in their genes as compared to other fish.

As we gain understanding of what makes the sea horse so special, its future is far from assured. Many of the 46 or so known species are on the endangered list: drained from the sea as by-catch and sent around the world as live pets or as dried food and medicine. The sea horse is a powerful symbol, and one that has been used to catalyse conservation efforts, such as the creation of protected marine zones in places such as the Philippines. But pollution and habitat loss are also taking their toll — as they are on much of the wider ocean environment. The white horses may still skim across the surface, but the world of Poseidon is losing its magic.